1. Field of the Invention
This invention relates to a fuel injection control apparatus for an internal combustion engine of a car, and particularly to such a control apparatus which processes measured values of the quantity of suction air in the internal combustion engine.
2. Background of the Invention
FIG. 1 illustrates a previously known fuel injection control apparatus for an internal combustion engine of the kind described above. Referring to FIG. 1, the numeral 1 designates an internal combustion engine. An electromagnetically driven injector (fuel injection valve) 2 supplies fuel to the internal combustion engine 1. A hot-wire air-flow sensor 3 detects the quantity of air sucked into the engine. A throttle valve 5 within a suction pipe 6 regulates the quantity of air sucked into the engine 1. A water temperature sensor 7 detects the temperature of the engine. A controller 8 computes the quantity of fuel to be supplied to the engine on the basis of an air quantity signal supplied from the air-flow sensor 3 and then applies a pulse having a width corresponding to the required fuel quantity to the injector 2. Further, an igniter 9 generates a pulse signal for the controller 8 at a predetermined rotational angle of the engine during each engine revolution. Also shown in FIG. 1 is a fuel tank 11. A fuel pump 12 applies pressure to the fuel in the tank 11. A fuel pressure regulator 13 maintains the fuel pressure to the injector 2 constant. Finally, there is shown an exhaust pipe 14. The controller 8 includes an input interface circuit 80, a microprocessor 81 and a ROM 82. The microprocessor 81 is arranged to process various kinds of input signals, to compute the quantity of fuel to be supplied through the suction pipe 6 to the combustion chamber as determined by the execution of a predetermined program stored in advance in the ROM 82, and to control a drive signal to the injector 2. A RAM 83 of the controller 8 temporarily stores data as the microprocessor 81 executes computations. An output interface circuit 84 of the controller drives the injector 2.
The conventional engine control apparatus operates as follows. The quantity of fuel to be supplied to the engine is calculated by the controller 8 on the basis of a suction air quantity signal detected by the air flow sensor 3. At the same time, the rotational frequency of the engine is calculated on the basis of a rotation pulse frequency obtained from the igniter 9, so that a fuel quantity per engine revolution can be calculated. The controller 8 applies a required pulse width to the injector 2 in synchronism with an ignition pulse. The pulse width applied to the injector 2 is corrected so as to be increased or decreased in accordance with a temperature signal generated from the water temperature sensor 7 because it is necessary to set the required air/fuel ratio of the engine to the rich side when the temperature of the engine is low. Further, the air/fuel ratio is made richer upon detecting engine acceleration by monitoring the opening of the throttle valve 5.
In the conventional apparatus as described above the use of the hot-wire air-flow sensor 3 makes it unnecessary to include means for correcting atmospheric pressure. This is so because the sensor 3 can detect the quantity of suction air by weight. However, the sensor 3 is sensitive to the return blow of air produced by valve overlapping of the engine so that it may detect a signal representing a quantity of suction air in which the quantity of the return-blow air is also included. Accordingly, the output signal generated by the air-flow sensor 3 may express a quantity of suction air which is larger than the actual quantity of the suction air. Return blow is apt to occur during low-speed, full-power operation of the engine. For example, as illustrated in FIG. 2, although the true suction air is not sucked during time t.sub.R, the measured suction air quantity has a wave form as shown in FIG. 2, which would seem to indicate that the suction air is increased by the return blow. As the result, the output of the air-flow sensor 3 expresses values, as shown in FIG. 3, considerably larger than the true values (shown by broken lines in the drawing), in the low-speed, full-power region. Although varying with the layout of the engine, the suction system, or the like, the error due to the return blow generally reaches a maximum of about 50% so that use of the sensor 3 as illustrated in FIG. 1 is not practical.
In order to compensate for such an error, there has been proposed a method in which values for the maximum quantity of suction air (including variations) to be sucked into the engine are stored in the ROM 82. As a result, as shown in FIG. 4, the output signal a generated from the air-flow sensor 3 is disregarded and clipped to a line of values as shown by "MAX" which are slightly larger (for example, 10%) than an average value b of the true suction air quantity. In this method, however, the clipping values represented by "MAX" imply that the maximum suction air quantity is set for engine operating conditions at sea level and at an ordinary temperature. Accordingly, the air/fuel ratio is greatly shifted to the rich side when the engine is operating at low atmospheric pressure at high altitudes or where the suction air temperature is high, increased fuel cost as well as the possibility of an accidental fire. Further, there is the corresponding problem that the air/fuel ratio is shifted to the lean side when the temperature of the suction air is low.
There also has been proposed a method in which wave forms affected by return blow are first determined and are then subjected to subtraction to thereby correct a detection error in a air-flow sensor 3 due to such return blow of suction air. However, the waveforms due to the return blow vary depending on both the rotational frequency of the engine and the opening of the throttle valve. Accordingly, it has been impossible to perform accurate correction.
Thus, with the conventional fuel injection control apparatus, there exists the problem that the hot-wire air-flow sensor 3 detects the suction air quantity as a value larger than the true value thereof because of the return blow of air produced during low-speed, full-power operation, so that the air/fuel ratio cannot be properly controlled over a certain running region.